During semiconductor device fabrication, layers of materials are formed over semiconductor substrates, e.g., wafers. Among the materials which can be included in such layers are tantalum pentoxide, titanium nitride, titanium silicon nitride, tantalum nitride, tantalum silicon nitride, titanium silicide, tantalum silicide, tungsten nitride, aluminum oxide, hafnium oxide, zirconium oxide, silicon nitride, silicon dioxide, elemental tungsten and elemental titanium. Methods for forming layers of such materials can include chemical vapor deposition (CVD) and atomic layer deposition (ALD).
Chemical vapor deposition includes mixing two or more reactants in a chamber to form a material which subsequently deposits across exposed surfaces of one or more semiconductor substrates. In CVD processes, it can be difficult to control reactions between the reactants provided in the chamber and various side-reactions can occur which can generate contaminants. Additionally, it can be difficult to form a uniform layer over multiple exposed surfaces of one or more semiconductor substrates with CVD. The deposition of CVD material can be faster in various regions of semiconductor topography than other regions, which can lead to within wafer (WIW) non-uniformity, e.g., increased WIW uniformity variance in a thickness of the deposited material across various exposed surfaces of semiconductor substrates provided within a CVD reaction chamber.
Atomic layer deposition (ALD) can overcome some of the problems discussed above relative to CVD. ALD processing includes forming thin films of material by repeatedly depositing monoatomic layers. The technique involves individually depositing reactants, e.g., precursors, that react in situ to form a desired film of material across a semiconductor substrate. More specifically, ALD processes involve introduction of a first reactant which reacts with a substrate to form a monolayer across the substrate. The first reactant will often react with the substrate, but not with itself. Accordingly, side-reactions can be reduced or eliminated. Further, the reaction of the reactant with the substrate can be self-limiting, e.g., once a monolayer forms across exposed surfaces of the substrate there is no longer further reaction of the reactant with the substrate.
In ALD processes, after the monolayer is formed, the excess first reactant can be evacuated from the reaction chamber via a purge process, and a second reactant can be subsequently introduced. A purge process can include one or more purge steps in which a purge gas, e.g., an inert gas, is introduced into the reaction chamber and one or more pumping steps preceding and/or following introduction of the purge gas to remove excess reactant, catalyst, purge gas, and/or by-product gases from the chamber.
In ALD processes, the second reactant reacts with the monolayer of material formed from the first reactant to convert such monolayer into a desired material layer over the substrate. The desired material layer can have a relatively uniform thickness across the various surfaces of the substrate, which can be made thicker by evacuating the second reactant from the processing chamber via a purge process and repeating the above-described process until a desired thickness of the desired material layer is formed.
Depending on the reactant system and with long enough pump and/or purge times, an ALD process can produce very uniform thickness across a wafer regardless of topography and can maintain uniform thickness profiles for each wafer in a batch if the processing temperature is held constant. However, the layer by layer ALD processing can have significantly lower throughput as compared to CVD processing techniques. To improve the throughput associated with ALD processes, the purge process can be shortened by using shorter pump and/or purge times between reactant pulses. In some cases, the deposition rate associated with ALD processing can be improved by increasing or decreasing the process temperature. Also, ALD throughput can be improved by processing a plurality of wafers simultaneously in a batch process.
However, performing batch processes, increasing or decreasing the process temperature, and/or shortening pump and/or purge times can lead to an added CVD component associated with an ALD process. An ALD process having an added CVD component refers to a quasi-ALD process which exhibits some CVD process characteristic, such as increased direct reactions between residual reactants and/or other CVD process characteristics, which can increase the WIW uniformity variance associated with the deposition process. For example, performing batch processes, increasing or decreasing the process temperature, and/or shortening the pump and/or purge time, e.g., the time used to evacuate the chamber between ALD reactant pulses, can lead to incomplete removal of the ALD reactants and thereby increases contaminants and/or co-reactions within the chamber.